Pulsed oscillator system



July 22, 1958 T. J. RYAN 844,725

' PULsED oscILLAToR SYSTEM FiledMay 24, 19,56

United States Patent C) PULSED oscrLLAToR SYSTEM Thomas J. Ryan,Langhorne, Pa., assignor to Philco Corporation, Philadelphia, Pa., acorporation of Pennsyl- .valia Application May 24, 1956, Serial No.586,965

8 Claims. (Cl. Z50-36) The present invention relates to signalgenerating systems and more particularly to systems for generating shortbursts or pulses of radio frequency energy.

Pulsed magnetrons have been used for a number of years to generate shortpulses of microwave frequency energy. Pulsed magnetrons are normallyoperated in a relatively strong magnetic ield, for example of the orderof 5,000 to 7,000 gauss. The amplitude of the pulse applied between theanode and the cathode to cause the pulsed magnetron to oscillate may beof the order of 5,000 to 20,000 volts. Special oxide coated cathodes areemployed in order to supply the high peak cathode current during thegeneration of the pulse. The peak output power of a pulsed magnetron isof the order of several kilowatts. Pulses having a duration of afraction of microsecond may be generated with tubes of the type justdescribed. Pulsed magnetron tubes are large and expensive. The modulatorcircuits required to drive these tubes are also large and expensive.Continuous wave or C. W.. magnetrons have been employed for a number ofyears to generate continuous oscillations at microwave frequencies. C.W. magnetrons normally operate with a relatively weak magnetic eld, forexample of the order of 1,000 gauss, and with a fixed anode potential ofthe order of 500 volts. The power output of a typical C. W. magnetron isof the order of one watt.

The anode structure and other physical characteristics of C. W.magnetrons are generally similar to the `anode structure andcorresponding physical characteristics of pulsed magnetrons. This fact,coupled with thel fact that the low power C. W. magnetrons and powersupplies of the order of 500 volts are smaller and cheaper than highpower pulse tubes and power supplies of the order of several kilovolts,has led to attempts to `construct low power pulse systems employing C.W. magnetrons as the microwave pulse generator. C. W. magnetrons havethe further advantage over pulsed magnetrons that they may be operatedat muchhigher duty cycles. Pulsed magnetrons usually are limited to dutycycles of the order of .001 while C. W. magnetrons may be operated withduty cycles `approaching 1.0. However, in many instances it has beenfound that C. W. magnetrons will not oscillate in response to shortduration pulses having amplitudes equal to or even slightly in excess ofthe normal operating potential of the tube. The inability of the C. W.magnetron to respond to pulses having a duration of a few microsecondsor less is believed to be due to the fact that the normal oscillationbuild up time in a C W. magnetron is generally rather long, for exampleof the order of a millisecond to several hundred milliseconds in somecommercially available C. W. magnetrons. This build up time should becompared to the oscillation build up time of a pulsed magnetron whichmay be of the order of a few millimicroseconds. The oscillation build uptime is believed to be related to the noise generating properties of themagnetron, and the fast'build up time of pulsed magnetrons is assumed toresult from the generationv of' random noise in response to the leadingedge of the modulating pulse. This random noise contains some componentsat the proper frequency to pre-excite the cavities of the pulsedmagnetron. For reasons which are not entirely clear, C. W. magnetronscannot be caused to exhibit a corresponding fast build up time withoutgoing to high magnetic ields and high peak anode Voltages. This cannotbe done without redesigning the tube to withstand the high peakvoltages, that is without rebuilding the C. W. magnetron into a pulsedmagnetron. It is not known whether this limitation applies to all C. W.magnetrons which are commercially available. but it is believed that itexists in most such tubes.

I have discovered a novel circuit which overcomes the above-describedlimitation of C. W. magnetrons and which permits C. W. magnetrons to beoperated as pulsed magnetrons at relatively low anode potentials andield strengths. The C. W. magnetrons may be operated over a wide rangeof duty cycles, for example from very low duty cycles such as .001 tovery high duty cycles approaching 1.0.

Therefore it is an object of the present invention to provide a novelcircuit for pulsing C. W. magnetron tubes.

A further object of the present invention is to provide a circuit inwhich the amplitude of the modulating pulse supplied to a C. W.magnetron may be less than the normal anode potential of the C. W.magnetron.

Still another object of the present invention is to provide a simpleeconomical circuit in which the oscillation build up time of a C. W.magnetron is greatly reduced.

A further object of the present invention -is to provide a simpleeconomical circuit for causing the oscillation build up time of a C. W.magnetron to approach that of a pulsed magnetron.

These and other objects of the invention are realized by providing acircuit which impresses a D. C. voltage, hereinafter referred to as theoscillation enhancement voltage, on the C. W. magnetron anode. Thisenhancement voltage is made less than the voltage which will cause theC. W. magnetron to oscillate.. A modulating pulse equal to or slightlygreater than the diierence between the normal anode potential of the C.W. magnetron and the enhancement voltage is superimposed on theenhancement voltage at such times as the C. W. magnetron is to generatemicrowave oscillations.

For a better understanding of the present invention together with otheryand further objects thereof reference should now be made to thefollowing detailed description which is to be read in conjunction withthe accompanying drawing in which:

Fig. l is a schematic diagram of a preferred form of pulse modulatorcircuit;

Fig. 2 is a plot showing a typical anode current vs. anode voltagecharacteristic for a C. W. magnetron; and

Fig. 3 is .a plot magnetron anode voltage as a function of time for thecircuit of Fig. 1.

In Fig. l the cathode of magnetron 12 is connected to ground. The anodeof magnetron 12 is connected toa source of anode supply potential,represented by the symbol B+, through the secondary winding 14 of apulse transformer 16. In the circuit of Fig. l it is preferable tooperate the anode above ground potential in order to minimize the numberof power supplies required. In many C. W. magnetrons the shell is notconnected to the anode so that it is possible to operatel the anodeabove ground potential without placing the shell above ground potential.If the particular C. W. magnetron employed has an exposed anode or aconnection between the anode and the shell and it is undesirable tooperate the anode or the shell above ground potential, it will usuallybe possible to follow the practice employed .in pulsed magnetrons andoperate the anode at ground potential and apply negative potentialstothe cathode.= The circuit modifications required will be obvious toanyone skilled in the art so no further explanation of this modicationwill be given herein.

The circuit shown for pulsing magnetron 12 comprises a pentode amplifiertube 18 which has the primary winding 20 of transformer 16 as an anodeload impedance. As shown in Fig. l the primary winding 20 is alsoreturned to the supply represented by the symbol .B+. The remainingconnections to tube 18 are conventional. The suppressor grid isconnected to the cathode. The resistor capacitor network 24 is providedfor maintaining the screen grid at a fixed positive potential. Aresistor capacitor coupling network 26 provides means for supplyingpositive pulses to the control grid of tube 18. The resistor in couplingnetwork 26 is returned to a negative bias source represented by theminus sign in Fig. l. The radio frequency signal generated by thecircuit of Fig. l appears at output connection 28 which is coupled tothe anode cavity of C. W. magnetron 12 in an appropriate manner.

The operation of the system of Fig. 1 will be explained with 'referenceto Figs. 2 and 3. As shown in Fig. 2, the relationship between the anodecurrent and anode voltage of magnetron 12 is represented by the curve32. Radio frequency oscillations are generated by magnetron 12 only ifthe anode potential is above the knee 34 in curve 32. It is generallyassumed that, for anode potentials below the knee 34, the magnetron willoperate in a so-called noise mode. That is, it is generally assumed thata magnetron operated below the knee 34 will generate random noisesignals which will appear at the output circuit even though themagnetron will not generate sustained oscillations at the low anodepotentials.

The presence of these noise signals is usually highly objectionable,particularly in pulsed type systems, since the noise generated in themagnetron may obscure received signals of small amplitude. However, Ihave discovered that, contrary to the usual assumption, many C. W.magnetrons do not generate noise signals even when the anode potentialis only slightly below the knee 34 of curve 32. I have also discoveredthat C. W. magnetrons which normally have a relatively long oscillationbuild up time, if pulsed from zero potential to a potential above= theknee 34 of curve 32 have a relatively short oscillation build up time,that is, a build up time of the order ot20 millimicroseconds if pulsedfrom a potential just below the knee 34 of curve 32 to a potential abovethe knee of this curve. Again this is true even though the C. W.magnetron may `not generate any appreciable noise signals for anodepotentials below the knee 34 of curve 32. Therefore, the anode supplypotential for magnetron 12 is selected to have a value slightly belowthe knee 34 of curve 32 as shown by the point on the voltage axislabeled B+.

Tube 18 of Fig. 1 is normally held below anode current cutol by thenegative bias supply associated with coupling network 26. However,positive pulses supplied by way of network 26, at the times that pulsesof radio frequency energy are to be generated by C. W. magnetron 12,cause tube 18 to conduct heavily. The conduction through primary winding20 induces a signal in secondary winding 14. The polarities of windings14 and 20 are so arranged that the potential appearing across winding 14is in a direction to add to the anode supply potential. Therefore theoperating point of magnetron 12 is moved to a point above knee 34 forthe duration of the pulse supplied by transformer 16.

Fig. 3 is a plot of the anode potential of magnetron 1 2 as a functionof time. The dashed line 40 represents the potential of the anode supplysource represented by the symbol B+. The broken line 42 represents theminimum anode supply potential necessary to generate sustainedoscillations. Curve 44 is a plot of the actual potential at the anode ofmagnetron 12 if pulses of time duration D are supplied to the controlgrid of tube 18 at times spaced apart by intervals T. It should be notedthat the pulses suppliedby winding 14 cause the anode need be set on themaximum 'amplitude of the anode' potential.

It has been observed that certain C. W. magnetrons exhibit a relativelysharp cathode saturation effect. This characteristic causes theamplitude of the oscillatory signal to be independent of anode potentialonce the anode potential exceeds the value which will cause saturation.The separation between the anode potential which will permit oscillationand the anode potential which will result in cathode saturation may beas small as l0 volts for some tubes. This effect causes the oscillatorypulses to have a constant amplitude throughout each pulse even thoughthe pulse supplied by transformer 16 is not a square-topped pulse.

It should be noted that the pulse supplied by winding 14 is only afraction of the total anode supply potential. In actual practice thepulse amplitude may be less than one-fifth of the potential representedby line 42. This fact makes it possible to employ a relatively low powertube in the pulse generating circuit. Ten watts of microwave power havebeen obtained using a subminiature tube for tube 18.

The oscillation present in curve 44 following each of the pulses is thewell known ringing effect caused by stray capacitance associated withtransformer 16. It has been found that this ringing does not result inthe generation of noise signals in the circuit of the present inventionprovided maximum amplitudes of the positive peaks do not approach thelevel represented by line 42. This ringing may be controlled bycontrolling the damping of transformer 16 or by other means well knownto persons familiar with pulsed magnetron circuits.

The power required from the pulsing circuit may be further reduced ifthe period of the oscillation or ringing in curve 44 is adjusted so thatthe pulse supplied by winding 14 occurs in time coincidence with apositive peak of the oscillation in anode potential. This is easilyaccomplished at relatively high repetition rates by designing the pulsetransformer to have the appropriate inductance and capacitance.

Y The presence of the steady D. C. potential on the anode of magnetron12 in the interval between pulses results in back-heating of the cathodeof magnetron 12. Therefore the filament power supplied to the magnetronmay be reduced as soon as anode potential is supplied to the tube.Insome types of C. W. magnetrons the filament power may be entirelyremoved once the cathode has reached its proper operating temperature.As the above description indicates, the circuit of Fig. l differs fromconventional pulse magnetron circuits in the presence of the D. C.supply connected to the anode of magnetron 12. Also, as pointed outabove, the advantages of this novel circuit have been made apparent byapplicants discovery that the oscillation starting time of a C. W.magnetron may be materially reduced if a steady D. C. potential, hereindesignated as the oscillation enhancement voltage, is supplied to themagnetron, and also the discovery that the application of theoscillation enhancement voltage would not cause the generation of noisein the output of the C. W. magnetron.

. A circuit having the following parameters has been tested and found tooperate in a highly satisfactory manner. However, it is to be understoodthat these parameters are given only by way of example and the inventionis not to be limited thereby.

While the invention has been described with reference to a singleembodiment thereof, it will be apparent that various modications andother embodiments thereof will occur to those skilled in the art withinthe scope of the invention. Accordingly I desire the scope of myinvention to be limited only by the appended claims.

What is claimed is:

l. A radio frequency pulse generator circuit comprising a magnetronhaving an anode and a cathode, said magnetron having the characteristicthat it generates substantially no radio frequency energy at anodepotentials below a first value and generates sustained oscillations atanode potentials within a selected range of potentials, the upper andlower limits of.said selected range being greater than said first value,a source of anode supply potential, a pulse transformer having a primarywinding and a secondary winding, the secondary winding of said pulsetransformer being connected in series circuit with said source of anodesupply potential, said series circuit being connected between said anodeand said cathode of said magnetron, the potential supplied by saidsource of anode supply potential being less than said first value, andmeans for supplying a signal to theprimary of said pulse transformer,thereby to produce a voltage pulse across said secondary winding, thesum of the potential supplied by said source of anode supply potentialand the potential appearing across said secondary winding of said pulsetransformer being equal to a potential in said selected range.

2. A radio frequency pulse generator circuit comprising a magnetronhaving an anode and a cathode, said magnetron having the characteristicthat it generates substantially no radio frequency energy at anodepotentials below a first value and generates sustained oscillations atanode potentials above a second value which is greater than said firstvalue, a source of anodesupply potential, a pulse transformer having aprimary winding and a secondary winding, the secondary winding of saidpulse transformer being connected in series circuit with said source ofanode supply potential, said series circuit being connected between saidanode and said cathode of said magnetron, the potential supplied by saidsource of anode supply potential being less than said first value, andmeans for supplying a signal to the primary of said pulse transformer,thereby to produce a voltage pulse across said secondary winding, thesum of the potential supplied by said source of anode supply potentialand the potential appearing across said secondary winding of said pulsetransformer being greater than said second value.

3. A radio frequency pulse generator circuit comprising a magnetronhaving an anode and a cathode, said magnetron having the characteristicthat it generates substantially no radio frequency energy at anodepotentials below a first value and generates sustained oscillations at asecond value of anode potential which is greater than said first value,a source of anode supply potential, a pulse transformer having a primarywinding and a secondary winding, the secondary winding of said pulsetransformer being connected in series circuit with said source of anodesupply potential, said series circuit being connected between said anodeand said cathode of said magnetron, the potential supplied by saidsource of anode supply potential being less than said first value, andmeans for supplying a signal Yto the primary of said pulse transl formerthereby to produce a voltage pulse across said secondary winding, thesum of the potential supplied by said source of anode supply potentialand the potential appearing across said secondary winding of said pulsetransformer being equal to said second value.

4. A radio frequency pulse generator circuit comprising a magnetronhavingan anode and a cathode,'said magnetron having the characteristicthat it generates substantially no radio frequency energy at anodepotentials below a first value and generates sustained oscillations atanode potentials above a second value which is greater than said firstvalue, a first source of anode supplypotential, a pulse transformerhaving a primary and a secondary winding, one terminal of said primarywinding and one terminal of said secondary winding being connected to afirst terminal of said source of anode supply potential, a secondterminal of said secondary winding being connected to the anode of saidmagnetron, a sec- Ond terminal of said source of anode supply potentialbeing connected to the cathode of said magnetron, an electron tubehaving at least an anode, a cathode and a -control grid, the anode ofsaid electron tube being connected to a second terminal of said primarywinding, the cathode of said electron tube being connected to saidsecond terminal of said source of anode supply potential and meansassociated with said control grid for periodically causing said vacuumtube to be rendered conductive, the potential supplied by said source ofanode supply potential being less than said first value, the sum of thepotential supplied by said source of anode supply potential and thepotential appearing across said secondary winding as a lresult of saidconduction through said electron tube being greater than said secondvalue.

5. A radio frequency pulse generator circuit comprising a magnetronhaving an anode and a cathode, said magnetron having the characteristicthat the anode current increases relatively slowly for unit increases inanode potential below a first value of anode potential and increasesrelatively rapidly for unit increases of anode potential above saidfirst value, said magnetron being further characterized vby the factthat said magnetron generates substantially no radio frequency energy atanode potentials below a rst value and generates sustained oscillationsat a second value of anode potential which is greater than said firstvalue, a source of anode supply potential, the potential supplied bysaid source being less than said first value by a small amount, a pulsetransformer having a primary winding, one terminal of said primarywinding and one terminal of said secondary winding being connected tothe positive terminal of said source of anode supply potential, meansconnecting the cathode of said magnetron to the negative terminal ofsaid source of anode potential, a second terminal of said secondarywinding being connected to the anode of said magnetron, an electron tubehaving at least an anode, a cathode and a control grid, theanode-cathode circuit of said electron tube being connected between asecond terminal of said primary winding and said second terminal of saidsource of anode supply potential and means associated with said controlgrid of said electron tube for rendering said electron tube periodicallyconductive, the sum of the potential supplied by said source and thepulse potential supplied by said secondary winding of said pulsetransformer during the intervals that said electron tube is conductingbeing equal to said second value.

6. A radio frequency pulse generator circuit comprising a magnetronhaving an anode and a cathode, said magnetron having the characteristicthat oscillations are generated only at values of anode potential abovea first value, a source of anode supply potential, the potentialsupplied by said source being less than said rst value by a smallamount, a source of substantially unidirectional 7 pulse signalsconnected in series with said source ofY anode supply potential, thepolarity of said pulse signals being such as to, increase directly thepotential appearing across the terminals of said series combination uponthe occurrence of each of said pulse signals, said series vcombinationbeing connected between said anode and said cathode of said magnetron,the sum of the potential supplied by said source of anode supplypotential and the peak potential of said pulse signals being at leastequal to said second value, whereby said magnetron is caused to generateoscillations in response to the application of each of said pulsesignals, said oscillations having a relatively rapid build-up rate.

7. A radio frequency pulse generator circuit comprisinga magnetronhaving an anode and a cathode, said magnetron having the characteristicthat the anode current increases relatively slowly for unit increases inanode potential below a first value of anode potential and increasesrelatively rapidly for unit increases of anode potential above saidlirst value, said magnetron being further charaeterized by the fact thatsaid magnetron generates substantially no radio frequency energy atanode potentials below a iirst value and generates sustainedoscillations at a second value of anode potential vwhich is greater thansaid first value, a source of anode supply potential, the potentialsupplied by said source being less than said first value by a smallamount, a source of substantially unidirectional pulse signals connectedin series with said source of anode supply potential, the polarity ofsaid pulse signals being such as to increase directly the potentialappearing across the terminals of said series combination upon theoccurrence of each of said pulse signals, said series combination beingconnected between said anode and said cathode of said magnetron, the sumof the potential supplied by said source of anode supply potential andthe peak potential of said pulse signals being at least equal to saidsecond value, whereby said magne- Y g tron is caused to generateoscillations in response to `the application of each of said pulsesignals, said oscillations having a relatively rapid build-up rate.

8. A radio frequency pulse generator circuit comprising a magnetronhaving an anode and a cathode, said magnetron having the characteristicthat the anode curt rent increases relatively slowly for unit increases'in an- `ode potential between a rst value of anode potential and asecond value of anode potential and increases relatively rapidly forunit increases of anode potential above said second value, saidmagnetron being further characterized bythe fact that it generatessubstantially no radio frequency energy at anode potentials below saidsecond value and generates sustained oscillations at a third value ofanode potential which is greater than said second value, a source ofanode supply potential, the potential supplied by said source being lessthan said second value by a small amount, a source of substantiallyunidirectional pulse signals connected in series with said source ofanode supply potential, the polarity of said pulse signals being such asto increase directly the potential appearing across the terminals ofsaid series combination upon the occurrence of each of said pulsesignals, said series combination being connected between said anode andsaid cathode of said magnetron, the sum of the potential supplied bysaid source of anode supply potential and the peak potential of saidpulse signals being at least equal to said third value, whereby saidmagnetron is "caused to generate oscillations in response to theapplication of each of said pulses, said oscillations having arelatively rapid build-up rate.

References Cited in the file of this patent UNITED STATES PATENTS2,507,351 Sherbatskoy May 9, 1950 2,572,707 Elie OCt. 23, 1951

